I've just installed SQL Developer 4 build 12.84. After running a few queries I notice the sql developer is using > 50% of the available CPU constantly forseveral hours, despite not obviously running any queries.

Well, thread dump analyzer says a high percentage of threads are sleeping (but none are waiting) on a monitor -- meaning they are either idle waiting for work or dependent on some external resource that is busy or not available. Not sure how to interpret this, since you say nothing obvious is running.

Nothing similar has been reported against 4.0 that I know of, but in the past a couple of possibilities (not necessarily related to your thread dump) have been:

It seems there are always changes with each release (even incremental Early Adopter releases) in the parser area, both in terms of new functionality and fixes. So if you are talking 4.0 EA2, perhaps this behavior changed in EA3, and will definitely change for production. Here is another discussion that may apply: